RFM22B/23B
6.7. Preamble Length
The preamble detection threshold determines the number of valid preamble bits the radio must receive to qualify a
valid preamble. The preamble threshold should be adjusted depending on the nature of the application. The
required preamble length threshold will depend on when receive mode is entered in relation to the start of the
transmitted packet and the length of the transmit preamble. With a shorter than recommended preamble detection
threshold the probability of false detection is directly related to how long the receiver operates on noise before the
transmit preamble is received. False detection on noise may cause the actual packet to be missed. The preamble
detection threshold is programmed in register 35h. For most applications with a preamble length longer than 32 bits
the default value of 20 is recommended for the preamble detection threshold. A shorter Preamble Detection
Threshold may be chosen if occasional false detections may be tolerated. When antenna diversity is enabled a 20-
bit preamble detection threshold is recommended. When the receiver is synchronously enabled just before the
start of the packet, a shorter preamble detection threshold may be used. Table 14 demonstrates the recommended
preamble detection threshold and preamble length for various modes.
It is possible to use RFM22B/23B in a raw mode without the requirement for a 101010 preamble. Contact customer
support for further details.
Table 14. Minimum Receiver Settling Time
Approximate Recommended Preamble Recommended Preamble
Mode
Receiver
Settling Time
1 byte
Length with 8-Bit
Detection Threshold
20 bits
Length with 20-Bit
Detection Threshold
32 bits
(G)FSK AFC Disabled
(G)FSK AFC Enabled
(G)FSK AFC Disabled +Antenna
Diversity Enabled
(G)FSK AFC Enabled +Antenna
Diversity Enabled
2 byte
28 bits
40 bits
1 byte
2 byte
—
—
64 bits
8 byte
OOK
2 byte
8 byte
3 byte
—
4 byte
8 byte
OOK + Antenna Diversity Enabled
Note: The recommended preamble length and preamble detection threshold listed above are to achieve 0% PER. They may
be shortened when occasional packet errors are tolerable.
6.8. Invalid Preamble Detector
When scanning channels in a frequency hopping system it is desirable to determine if a channel is valid in the
minimum amount of time. The preamble detector can output an invalid preamble detect signal. which can be used
to identify the channel as invalid. After a configurable time set in Register 60h[7:4], an invalid preamble detect
signal is asserted indicating an invalid channel. The period for evaluating the signal for invalid preamble is defined
as (inv_pre_th[3:0] x 4) x Bit Rate Period. The preamble detect and invalid preamble detect signals are available in
"Register 03h. Interrupt/Status 1" and “Register 04h. Interrupt/Status 2.”
6.9. Synchronization Word Configuration
The synchronization word length for both TX and RX can be configured in Reg 33h, synclen[1:0]. The expected or
transmitted sync word can be configured from 1 to 4 bytes as defined below:
synclen[1:0] = 00—Expected/Transmitted Synchronization Word (sync word) 3.
synclen[1:0] = 01—Expected/Transmitted Synchronization Word 3 first, followed by sync word 2.
synclen[1:0] = 10—Expected/Transmitted Synchronization Word 3 first, followed by sync word 2, followed by
sync word 1.
synclen[1:0] = 1—Send/Expect Synchronization Word 3 first, followed by sync word 2, followed by sync word 1,
followed by sync word 0.
The sync is transmitted or expected in the following sequence: sync 3sync 2sync 1sync 0. The sync word
values can be programmed in Registers 36h–39h. After preamble detection, the part will search for sync for a fixed
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